A radio node typically establishes a connection to a radio communication network using contention-based random access. Under contention-based random access, different radio nodes contend for access to the radio communication network without coordinating their access with one another, e.g., the access attempts are random and contentious. A radio node that detects its random access attempt has collided with another radio node's attempt re-attempts access, e.g., after waiting for some period of time.
Establishing a connection to a radio communication network using random access often involves a radio node establishing the proper transmission timing, so that its transmissions can be recovered by the network. A radio communication network in this regard controls the timing with which radio nodes transmit to a common destination, so that those transmissions arrive at the common destination at roughly the same time; that is, the radio nodes' transmissions arrive synchronized or time-aligned. A radio communication network does this by controlling the so-called transmission advancement timing of each radio node. A radio node's transmission advancement timing describes the amount of time, if any, that a radio node advances its transmission in time, in order for that transmission to arrive time-aligned with other nodes' transmissions, e.g., accounting for the different propagation delays of the different transmissions. Synchronizing transmission reception in this way helps maintain orthogonality between different nodes' transmissions and thereby mitigates inter-node interference.
Given the contentious nature of random access and the need for establishing transmission timing, conventional random access approaches such as those currently employed by Long Term Evolution (LTE) networks require four steps. The first two steps require the radio node to perform a transmission and the network to respond with a timing advance that establishes the radio node's transmit timing. With the proper transmit timing established, the last two steps require the radio node to perform another transmission and the network node to respond again, in order to resolve any contention that may have occurred in the first two steps.
Some contexts challenge the ability of conventional random access approaches to establish transmission timing in the face of contention, in a time-efficient manner. When performed over unlicensed radio spectrum, for example, each separate transmission in the random access procedure delays the procedure because the unlicensed radio resources must be cleared of transmission activity before that transmission occurs.